Implement generic dummy mixer

Is your feature request related to a problem? Please describe. I route librespot into pulseaudio to mix it with other sources. The result is written into a socket which is consumed by snapcast. From a users perspective it would be nice if the volume slider in Spotify/the Spotify App could control the general master volume of snapcast.

Describe the solution you'd like I would like to control the Spotify volume slider without adjusting the actual volume. The volume regulation should happen by an external script which receives the selected volume (I think onevent already receives the selected volume) and e.g. does http requests to something to adjust the volume.

Describe alternatives you've considered I considered (ab)using pulseaudio for that purpose. Unfortunately it seems like the PA backend is not supported for the --mixer option. I also tried to make use of --mixer softvol --volume-ctrl fixed --initial-volume 100 (Which is described here) but this just disabled volume control via Spotify completely and therefore an onevent-script cannot receive the volume change.

Additional context Supporting pulseaudio as mixer might be a viable solution but I think a much more generic solution would be a "dummy" volume regulation which receives the selected level but does not adjust the audio stream.